1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a film containing an organic compound (the film is hereinafter referred to as organic to compound layer), and to a method of manufacturing the light emitting device. Incidentally, in the present specification, the light emitting device indicates an image display device or a plane emission device using a light emitting element as a light emitting element. Besides, the light emitting device includes a module in which a FPC (Flexible Printed Circuit), a TAB (Tape Automated Bonding) tape, or a TCP (Tape Carrier Package) is attached to a substrate over which a light emitting element is formed, a module in which a printed wiring board is provided at an end of a TAB tape or a TCP, and a module in which an IC (Integrated Circuit) is directly mounted on a substrate over which a light emitting element is formed by a COG (Chip On Glass) system.
2. Description of the Related Art
Light emitting elements, which employ organic compounds as light emitting member and are characterized by their thinness and light weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the organic compound layer to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is large, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
An active matrix type light emitting device structured by that a thin film transistor (hereinafter, referred to as TFT) is formed on the insulating surface, an interlayer insulating film is formed thereon, and that the first electrode of a light emitting element connected electrically to the TFT through the interlayer insulating film.
In addition, an organic compound layer is formed on the first electrode. The organic compound layer includes a hole injection layer, a hole transporting layer, a light emitting layer, a blocking layer, an electronic transporting layer, and an electronic injection layer. The organic compound layer can be formed by a single layer, however, it can also be formed by combining above-mentioned plural layers. A light emitting element is formed by that the second electrode is formed after that the organic compound layer is formed.
When these electrodes are anodes, metals having a large work function are used as an electrode material for an improvement of hole injection from an anode to an organic compound layer. It is noted that a work function of ITO is 4.8 eV, that is made frequently use as an anode material. On the other hand, in the case that these electrodes are a cathode, small work function metals or alloy containing said metals are respectively used for an improvement of electron injection from a cathode to an organic compound layer, because a great deal of organic compound has a small electron affinity in comparison with metals or inorganic semiconductors. Typically, as metals having small work function, one is belonging Group 1 or Group 2 in periodic table of the element is preferable to be used.
In an active matrix type light emitting device, in the case that a luminescence is extracted from a light emitting element connected electrically to TFT on the substrate across the substrate, (for example, unexamined patent publication Nos. 6-325869, 7-153576, and 8-241047), it is limited to the region where the light emitting region is formed in each pixel of TFT, wiring and the like. Therefore, it produces a problem that a rate of light emitting region (an aperture ratio) having a place in the pixel region become small.
In order to solve the problem, in the case that light generated from a light emitting element is emitted from a face opposed to the substrate where the light emitting element is formed thereon (hereinafter, referred to as an upward emission), for example, it is disclosed in unexamined patent publication Nos. 7-111341, 8-54836, and 10-189252, problems due to an aperture ratio can be prevented.
However, in any case of above-mentioned structures, the first electrode electrically connected to a TFT is formed on the interlayer insulating film covering TFT, so that the face has irregularities. At this time, an organic compound layer of a light emitting element with a thickness of 20 to 200 nm formed on an electrode is so thin that make a difference in level when an electrode is formed, and that result to produce a defective deposition in the organic compound layer formed on the difference level. The portion of a defective deposition is cause to short-circuit between the first electrode forming a light emitting element and the second electrode formed on the organic compound layer. Therefore, there are some cases that the method structured by that the edge portion of the first electrode is covered by an insulating film is adopted.
In addition, in the forming step of the second electrode, in the particular case that the light is emitted from the second electrode side, the thickness should be thin not to deteriorate the transmittance. Similarly, a level difference due to the first electrode may be a cause of deposition deterioration in forming of the second electrode.
A transparent conductive film needs to be formed above the organic compound layer in order to emit light from opposite face to the substrate. However, a problem is occurred that necessity of high energy to form the film is severely damage the organic compound layer.